Despite advances in the understanding of underlying mechanisms, heart failure (HF) remains the major cause of mortality, indicating the urgent need in the development of novel unconventional therapeutic strategies. Abnormal intracellular calcium (Ca2+) handling has been implicated in the pathogenesis of malignant arrhythmias characteristic of Heart Failure (HF). Cardiac excitation-contraction (EC) coupling is mediated through Ca2+-induced Ca2+ release (CICR), which is controlled by sarcoplasmic reticulum (SR) Ca2+-sensitive Ca2+ channels, also known as cardiac ryanodine receptors (RyR2). MicroRNAs (miRs) are ~22-nucleotide-long nonprotein-coding RNAs that recognize their target mRNAs by base pairing interactions and subsequently inhibit gene expression by targeting these mRNAs for translational repression or degradation. Rapidly accumulating evidence implicates dysregulated miRs in cardiac pathogenesis including arrhythmias and HF rendering them new attractive targets for therapy. However, much work must be done for better understanding of miR functions. Our preliminary results implicate a family of miRs specific to muscle tissue called myomiRs in regulation of Ca2+ handling in cardiomyocytes. The central hypothesis of this proposal is that myomiRs which include miR-208a, miR-208b and miR-499 play a critical role in regulating Ca2+ homeostasis by modulating the structure and function of macromolecular complexes involved in Ca2+ handling. In order to test this hypothesis we have developed techniques to modify the expression of miRNAs in vitro and in intact hearts in vivo in order to examine their effects on Ca2+ cycling in single ventricular myocytes using electrophysiology and single photon laser scanning confocal microscopy. Specifically we aim to investigate the molecular determinants of myomiR-mediated regulation of SR Ca2+ release through RyR2s in ventricular myocytes, and test whether aberrant expression patterns of these miRs contribute to Ca2+-dependent arrhythmias characteristic of cardiac disease using rat model of heart failure.